The invention relates to a multiscrew, continuous mixing machine for plasticizable compounds with two screws, which rotate in the same direction in a housing and which penetrate the cylindrical part of the housing that envelopes the screw concerned and exhibit feed and metering sections and inbetween at least one homogenizing section and a mixing section, whereby the feed and metering sections are formed by intermeshing conveying screws and are arranged in the mixing elements, which are opposite the homogenizing and mixing sections, followed by restricting elements, which consist of short screw elements with back feed thread.
Such a machine is described in the DE-PS 43 38 795 (=U.S. Pat. No. 5,499,870). In the case of this machine, the restricting elements following the mixing elements ensure that a chamber which extends above the mixing elements is constantly filled, whereby it is supposed that the machine is constantly fed an appropriate quantity of the material to be mixed. In the case of the known machine, the restricting elements consist of short screw elements with interrupted back feed thread. The interruptions in the area of the vertex of the back feed thread produce overflow channels as it were, which owing to their size, enable the plasticized material to flow.
Owing to the interruptions in the back feed thread, a constant flow opening is provided, which is independent of the speed at which the machine is operated, which results in an identical pressure always being present in the chamber extending above the mixing elements, which is essentially independent of the speed.
The invention is based on being able to provide a selectable variable pressure in the chamber. By means of the invention, this takes place in that the back feed thread forms a vertex, which progresses radially shortened along a thread in such a way that the radius of the vertex is shorter on the inlet side than on the outlet side and inbetween increases continuously towards the outlet side.
This arrangement of the vertex of the back feed thread results in the surprising effect that the pressure in the chamber increases with increasing speed. The machine operator is therefore able to optionally adjust the pressure in the chamber by varying the speed of the two screws of the machine, which can be used to exert a particular influence on the material to be plasticized. This results in the quantity of energy supplied to the material to be plasticized increasing with increasing pressure in the chamber, which depending on the viscosity of the material concerned, is expressed in an appropriate temperature in the chamber. In addition, there is also the effect that the additives added to the material to be plasticized, more or less intensively intermix with the material.
Based on this simple measure of optional adjustment of the machine speed, a broad spectrum of various materials can be processed with this machine without having to alter the configuration of the machine elements, particularly the restricting elements, which would make it necessary to disassemble and reassemble the machine. For adjustment to processing different materials, according to the invention, it is only necessary to vary the speed, which is easily possible by appropriate control of the usual electric drive motor. According to the invention, the machine is suitable for virtually universal application.
The effect of the increasing pressure in the chamber with increasing speed can be explained in that in contrast to the aforementioned interruptions in the feed back thread of the restricting elements, which according to the invention continuously produces a special dispersing effect in the radius of the thread vertex increasing in the direction of passage through the machine, which apparently becomes more pronounced the higher the machine speed, so that at extremely high speeds, virtual sealing of chamber results due to the restricting elements, which intermesh at the end of their threads on the outlet side of the chamber and thus form a complete seal. The thread region with relatively short radius allows the material to flow through, however, greatly reduced at higher speed, apparently because at higher speed, the material is moved in backward feeding direction more quickly the higher speed as a result of the radius of the vertex enlarging in conveying direction.
For practical reasons, the radius of the vertex is designed so that the restricting elements opposite the vertexes on the inlet side travel past each other and subsequently ascend to the extent that the vertexes on the outlet side fully intermesh.
It has shown that with such a design of the vertex, the influence of the machine speed is particularly pronounced.
According to the invention, it is possible to provide the restricting elements with varying vertex radius in the area of the vertex, interruptions in the form of radial depressions, if a particularly high throughput in the region of the restricting elements is required or necessary.
For practical reasons, one combines the developed restricting elements according to the invention with preceding mixing elements, which are also developed in the nature of conveying screws, however, with such pitch direction that one mixing element within the chamber conveys backwards.